**Core Concept**
Glycogen metabolism is a complex process involving the breakdown and synthesis of glycogen, a branched polysaccharide stored in the liver and muscles. This process is crucial for maintaining blood glucose levels, particularly during fasting periods. The key enzymes involved in glycogen metabolism are glycogen synthase, glycogen phosphorylase, and debranching enzyme.

**Why the Correct Answer is Right**
Glycogen synthase is the primary enzyme responsible for glycogen synthesis. It catalyzes the transfer of glucose-1-phosphate to the growing glycogen chain, forming an alpha-1,4-glycosidic bond. This process is essential for glycogen accumulation in the liver and muscles. In contrast, glycogen phosphorylase is responsible for glycogen breakdown, catalyzing the phosphorolysis of glycogen to glucose-1-phosphate. The debranching enzyme is involved in the breakdown of branch points in glycogen, allowing for the continued phosphorolysis of glycogen.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the correct enzyme involved in glycogen metabolism.
* **Option B:** This option is incorrect because it refers to an enzyme involved in glycolysis, not glycogen metabolism.
* **Option C:** This option is incorrect because it refers to an enzyme involved in gluconeogenesis, not glycogen metabolism.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that glycogen storage diseases, such as glycogen storage disease type I (GSD I), result from defects in enzymes involved in glycogen metabolism. These diseases can lead to severe hypoglycemia and lactic acidosis.

**Correct Answer:** C. Glycogen synthase is the primary enzyme responsible for glycogen synthesis.